Both of Tay-Sachs disease and Sandhoff disease are diseases exhibiting neurological symptoms which are caused by accumulating GM2 ganglioside in neural system cells due to decreased activity of β-hexosaminidase A (Hex A). Hex A is a heterodimer composed of an α-subunit and a β-subunit and has the enzyme activity to degrade GM2 gangliosides. Tay-Sachs disease is a Hex A deficiency caused by α-subunit deficiency and Sandhoff disease is a Hex A deficiency caused by β-subunit deficiency.
The present inventors have previously provided a cell line which was established by introducing an expression vector into which genes encoding an α-subunit and a β-subunit (HEXA cDNA and HEXB cDNA, respectively) are inserted, into CHO cell strains or specific yeast strains and which constitutively expressed a wild-type recombinant Hex A. The present inventors found that the administration of thus produced wild-type recombinant Hex A to Sandhoff disease model mice leads to the reduction of GM2 gangliosides accumulated in neural system cells and the improvement of neurological symptoms, and thus confirmed that an enzyme replacement therapy is efficacious for Tay-Sachs disease and Sandhoff disease (Patent Document 1 and Non-Patent Document 1).
However, in general, in the case where a therapeutic agent containing an enzyme which is deficient is repeatedly administered to a patient, the enzyme in the therapeutic agent is often recognized as a foreign matter in the patient and thus the antibody is produced. Consequently, adverse/side reactions such as allergy reaction and anaphylactic reaction are occurred in the patient. Therefore, in the case where a wild-type recombinant Hex A is directly administered to a patient with Tay-Sachs disease or Sandhoff disease, adverse/side reactions may be occurred in the patient similarly as described above. Furthermore, a wild-type recombinant Hex A has disadvantages such as low stability in blood (plasma) and low ratio of uptake into cells of a disordered organ (neural system cells).
In order to solve these problems, the present inventors produced a modified β-subunit in which the active site of a β-subunit is substituted with the active site of an α-subunit based on conformational information of the α-subunit and β-subunit of β-hexosaminidase. Furthermore, the present inventors produced a modified β-hexosaminidase B which is a homodimer composed of the modified β-subunit (hereinafter, referred to “ModB”) and confirmed that the recombinant enzyme has the activity to degrade GM2 gangliosides (Patent Document 2 and Non-Patent Document 2).